Resetting Apple Watch Series 7 passcode without the paired iPhone

Forgot series 7 i watch passcode and it’s paired to my old I phone i no longer have. How do i totally reset my i watch if I don’t have my old phone that it is paired to? I need to sell it because I am getting the series 10 I watch, but I can’t sell it to the new user because I can’t remember the passcode. What can I do??!!

[Re-Titled by Moderator]